Vertebral compression fractures in osteoporotic patients are typically treated by a surgical procedure known as vertebroplasty. In this procedure the fractured vertebral body is augmented with a bone cement. The bone cement polymerizes and hardens upon injection into the vertebral body and stabilizes the fracture. Pain relief for the patient is usually immediate and vertebroplasty procedures are characterized by a high rate of success.
Typically, bone cement is prepared directly prior to injection by mixing bone-cement powder (e.g., poly-methyl-methacrylate (PMMA)), a liquid monomer (e.g., methyl-methacrylate monomer (MMA)), an x-ray contrast agent (e.g., barium sulfate), and an activator of the polymerization reaction (e.g., N, N-dimethyl-p-toluidine) to form a fluid mixture. Other additives including but not limited to stabilizers, drugs, fillers, dyes and fibers may also be included in the bone cement. Since the components react upon mixing, immediately leading to the polymerization, the components of bone cement must be kept separate from each other until the user is ready to form the desired bone cement. Once mixed, the user must work very quickly because the bone cement sets and hardens rapidly.
Other examples of bone cement compositions and/or their uses are discussed in U.S. Pat. No. 7,138,442; U.S. Pat. No. 7,160,932; U.S. Pat. No. 7,014,633; U.S. Pat. No. 6,752,863; U.S. Pat. No. 6,020,396; U.S. Pat. No. 5,902,839; U.S. Pat. No. 4,910,259; U.S. Pat. No. 5,276,070; U.S. Pat. No. 5,795,922; U.S. Pat. No. 5,650,108; U.S. Pat. No. 6,984,063; U.S. Pat. No. 4,588,583; U.S. Pat. No. 4,902,728; U.S. Pat. No. 5,797,873; U.S. Pat. No. 6,160,033; and EP 0 701 824, the disclosures of which are herein incorporated by reference.
The elastic moduli of typical PMMA bone cements lie around 2-4 GPa, while the elastic modulus of osteoporotic cancellous bone lies in the range of 0.1-0.5 GPa. This mismatch in stiffness is generally perceived as favoring the subsequent fracturing of the vertebral bodies that are adjacent to the augmented vertebral body.
It is therefore an object of the invention to obtain a bone cement with a reduced stiffness that is adapted to the stiffness of the surrounding bone. This is thought to be an efficient way to reduce the risk of adjacent vertebral body fractures after the augmentation of vertebral bodies.
Reduction of the stiffness by introducing non-miscible phases, such as aqueous
components, into the PMMA upon polymerization is well known and has been described before. This leads to a macroporous structure with reduced stiffness.